Threshold voltage extraction circuits are important in various applications, for example, metal oxide semiconductor field effect transistor (MOSFET) process monitoring, device characterization, temperature sensing and voltage reference generation, based on its high linearity with temperature. A number of prior art circuits providing this function either have the shortcoming of requiring a twin-well process, or they are sensitive to power supply variation.
FIG. 1 shows a typical prior art threshold voltage extraction circuit. By driving n-type metal oxide semiconductor (NMOS) transistors M1 and M2 into saturation, the currents flowing in the two transistors, I1 and I2, respectively, are equal, if one neglects channel length modulation effects. Thus:I1=K1(VGS1−VTH1)2=I2=K2(VGS2−VTH2)2,  Eq. (1)where VGSi is the gate-to-source voltage of transistor Mi, VTHi is the threshold voltage of transistor Mi, Ki=Kp(W/L)i of transistor Mi, and Kp=μoCox. From theory, μo is the average electron mobility in the channel, and Cox is the gate oxide capacitance per unit area, for a given transistor.
By choosing K1=4K2, or sizing the transistors such that (W/L)1=4(W/L)2, and assuming that VTH=VTH1=VTH2, the threshold voltage of the NMOS Device Under Test (DUT) can be expressed as:
 VTH=2VGS1−VGS2,  Eq. (2)
By fixing the gate bias VGS2, and by using a current mirror circuit for the current sources for I1 and I2, the gate bias VGS1 is automatically adjusted to satisfy Equation (2).
However, most prior art approaches to implementing such an arrangement use a stacked transistor array for the gain-of-two (X2) amplifier 102. The disadvantage of this is that a twin-well process is required to implement the stacked transistor array, which adds cost. In addition, a subtractor-transistor network or an instrumentation amplifier is typically used to provide the function of subtractor 101. This adds to the complexity of the circuit which, again, adds cost.
Therefore, it would be desirable to provide a threshold voltage extraction circuit which overcomes the problems of the prior art.